Atomic layer deposition apparatus are known from, for example, WO2009/142487 and WO2012/005577. WO2009/142487 discloses an atomic layer deposition apparatus for continuous processing of semiconductor substrates, wherein the substrates are supported by gas bearings. The disclosed apparatus comprises a process tunnel that extends in a longitudinal direction and that is bounded by at least two walls. The walls are mutually parallel and spaced apart, so that a substantially flat substrate may be accommodated parallel between them. The walls of the process tunnel are provided with a plurality of gas injection channels to provide the gas bearings to floatingly support the substrate in the process tunnel.
The gas injection channels in at least one of the first and second walls are successively connected to a first precursor gas source, a purge gas source, a second precursor gas source, and a purge gas respectively. The successively connected gas injection channels create an atomic layer deposition-segment that—in use—comprises successive zones including a first precursor gas, and purge gas, a second precursor gas and a purge gas, respectively. At least two of such tunnel segments are disposed in succession in the transport direction as can be seen in FIG. 1. The segments form an atomic layer deposition apparatus for successively applying layers of chemically deposited precursor reaction products. In order to provide the forward movement to the substrate that is required for continuous processing, the apparatus is provided with a downwardly sloping process tunnel. The downward slope enables gravity to drive the floatingly supported substrates through the successive segments, causing atomic layers to be deposited on the substrates during transportation through the process tunnel in a transport direction.
The (forward) movement of these substrates in a transport direction, especially at higher velocities, may cause a fraction of the precursor gas used in a first precursor zone to be pushed or dragged forwardly, thereby traversing a downstream purge gas zone into a second precursor zone, causing mixing of the precursor gases from both zones. Since the precursor gases are mutually reactive, such mixing results in unwanted chemical vapor deposition in the process tunnel. This may, for example, lead to clogging of the gas injection openings and lateral gas exhaust openings in the deposition and purge gas zones of the process tunnel. A relatively high amount of maintenance and cleaning time is required to remove the depositions in the segments of the process tunnel. This in turn leads to a (significant) negative impact on the uptime of the apparatus and consequently, a reduced production capacity.
The traversal of the precursor gases originates from two sources. First of all, the gas disposed in a segment in front of the substrate is displaced in the transport direction by the moving substrate. The displacement causes the gas to traverse from a (first) precursor zone through a purge gas zone into an adjacent, successive (second) precursor gas zone of the process tunnel. An example of such traversal effect is provided in FIG. 2, which shows a schematic view of water vapor (H2O)—being a precursor gas—traversing the nitrogen (N2)-purge gas zone (+1N2, +2N2, +3N2) into the tri-methyl aluminum (TMA)-precursor gas zone.
Secondly, the presence of a substrate in the process tunnel produces a high pressure region above and below the substrate compared to the open region in the process tunnel between the substrates. The resulting pressure gradient causes a flow of gas from the relatively high-pressure regions to the relatively low-pressure regions in front of and behind the substrate. Consequently, a gas flow into the adjacent segments of the process tunnel may be created, which is caused by the presence of a substrate. Consequently, the high and low pressure region ‘travel’ along with the movement of the substrate. The resulting gas flow, for example comprised of precursor gas injected in a first precursor gas injection zone may under influence of the pressure gradient traverse the purge gas zone into a second precursor gas zone. An example thereof is shown in FIG. 3.
As a result of the abovementioned causes, at least part of a precursor gas may traverse the purge gas zone to subsequently mix with a second, different precursor gas in a second precursor zone.